The publication BALL GRID ARRAY TECHNOLOGY, John H. Lau, published by McGraw-Hill, Inc., 1995, describes the growing popularity of ball grid array (BGA) technology, indicating the many advantages over, for example, pin grid array (PGA) technology as follows: reduced coplanarity problems (no leads); reduced placement problems (self-centering); reduced paste printing problems; reduced handling issues (no damaged leads); lower profile (smaller size); better electrical performance; better thermal performance; better package yield; better board assembly yield; higher interconnect density; cavity-up or -down options; multilayer interconnect options; higher number of inputs/outputs (I/Os) for a given foot-print; shorter wire bonds; easier to extend to multichip modules; and faster design-to-production cycle time.
In a typical plastic ball grid array device (PBGA) 10 (FIG. 2), a printed circuit board (PCB) such as bismaleimide triazine (BT) resin or ceramic (Al.sub.2 O.sub.3) is used as a substrate 12. A silicon integrated circuit (IC) chip is provided on one side of such a substrate 12, with solder balls on the opposite side thereof, and with a molding compound 14 encapsulating the IC chip.
Electrical connection is made from the chip to the solder balls by means of wire bonding of flip chip connecting the chip to conductors on the surface of the substrate, from the conductors to traces and then through vias to the opposite side of the substrate on which other traces are provided, and then to the solder balls.
Up to the present time, BGA technology has been implemented in production mainly in high I/O application. For example, current BGA devices widely available in the semiconductor industry have 119, 169, 225, 256, 313, 352, 420 or 625 balls. Although devices with a lower number of I/O's make up the great bulk of the semiconductor product line at present, producing such lower number I/O devices in BGA technology has proven to be expensive. If BT is used, a large part of this expense, i.e., for example perhaps 50% of the cost of materials of the entire device 10, is tied up in the BT substrate 12.
Typically, BT or ceramic is provided in single-element form 16 (FIG. 1) with dimensions of, for example, 45 mm by 187.5 mm. It then falls to the manufacturer of BGA devices to lay out such devices in an attempt to use the maximum area of the element 16. Then, the completed devices 10 are singulated (indicated by the dotted lines 17 on the element 16), with excess material cut away, to result in individual BGA devices 10 (FIG. 2).
It will readily be seen that a substantial part of the element 16 is unused in the fabrication of devices 10, and is thus wasted. In fact, typically, in the current state of the industry, only 60 to 80% of the area of the element 16 is actually used in the fabrication of devices 10, while the rest is discarded.
As pointed out above, the material of element 16 is a significant part of the cost of the materials of the device. It is therefore of great interest to minimize that portion of the element 16 which is not used in the actual fabrication of BGA devices, so that overall device cost can be reduced, in turn allowing the industry to move toward profitable production of devices of lower I/O count.